1. Field of the Invention
The present invention relates to a control device for a die cushion mechanism which generates a force, on a slide of a press machine, using a servomotor as a drive source and, in particular, to a control device for controlling the force generated by the die cushion mechanism of the press machine.
2. Description of the Related Art
It is known that a press machine, for press working such as bending, drawing or punching, is provided with a die cushion mechanism, as an attached device, for applying a predetermined force or pressure, during the press working, to a movable support member (generally called a slide) supporting a first mold for press working, the force being generated by another movable member supporting a second mold. The die cushion mechanism is generally configured such that the slide (or the first mold), moving in a mold-clamping direction, directly or indirectly collides with a movable element (generally called as a cushion pad) held at a predetermined pressure and, until the molding is finished, the cushion pad is moved with the slide while applying force or pressure to the slide. During this operation, it is possible to prevent the occurrence of wrinkles in a workpiece to be pressed by, for example, clamping an area around a site, of the workpiece, to be pressed between the cushion pad and the slide.
Many conventional die cushion mechanisms use hydraulic or pneumatic units as driving sources. However, control by a hydraulic or a pneumatic unit may be carried out only under a constant pressure. It is preferable that the pressure during drawing is varied in response to the amount of the drawing, however, the pressure cannot be varied in the hydraulic or the pneumatic unit.
In recent years, therefore, a die cushion mechanism using a servomotor as a driving source has been used to carry out force control, with a fast response, as described in Japanese Unexamined Patent Publication (Kokai) No. 10-202327. In the die cushion mechanism described in this publication, a cushion pad positioned below a slide of a press machine may be upwardly and downwardly moved by a servomotor, corresponding to the rise and fall motions of the slide. In this configuration, the pressure is controlled by applying a free curve, predetermined corresponding to the mold, to a curve of a relation between the pressure of the cushion and the stroke of the cushion corresponding to the position of the cushion pad. Further, Japanese Patent Publication (Kokoku) No. 7-106477 discloses an example in which the pressure of the cushion is controlled according to a load pattern, including a predetermined pressure on the cushion, corresponding to the position of the slide.
In controlling the servo die cushion in the prior art, a force signal for controlling a force or a pressure is necessary and, moreover, a force detector for detecting a force, for generating the force signal, is necessary. However, measurement by the force detector is easily affected by vibration or the like due to rapid acceleration or deceleration of the slide associated with a speeding-up of the motion of the machine. Therefore, the detected force value may include a significant error, whereby the force cannot be exactly controlled.